Fabrication of a composite substrate for use in applications in the fields of optics, opto-electronics, or electronics often involves the need for transferring a layer of a material from a source substrate onto a support substrate. The methods for transferring a layer of material from a source substrate to a support substrate generally include applying the source substrate and support substrate against each other by molecular bonding, i.e., without using adhesive or an adhesive film. The transfer of the layer is possible provided that the mechanical retention of the layer on the source substrate is lower than the mechanical retention of the layer on the support substrate. This often is achieved by providing a weakened or detachment zone between the transferable layer and the source substrate, which zone allows detachment of the transferable layer without loss of the molecular bond between the transferable layer and the support substrate.
When adhesive is used for applying the source substrate to the support substrate, however, transferring the layer becomes more difficult since the exact volume of adhesive deposited on the substrate is difficult to control. Thus, the adhesive very often projects out beyond the respective side faces or sides of the substrates to form a flash so that the periphery of the area adjacent the detachment zone is covered at least partially and often completely by the adhesive. This occurs because the transferable layer is relatively thin. In addition, since the adhesive is cured to obtain the desired adhesive strength, the adhesive flash is also cured and hardened
When this occurs, it becomes very difficult to properly detach and transfer the transferable layer. For example, the mechanical force to be applied to detach the layer becomes very high, and this may result in cleavage of one of the substrates in areas where not intended. In particular, cleavage or splitting of the support substrate may occur along fracture lines which no longer extend in a uniform plane, such as that provided by the detachment zone, but instead in a random and unpredictable manner. This causes additional polishing or other treatment steps before the surface of the transferable layer or source substrate can be used in further processing steps to form the optic, opto-electronic, or electronic component.
Thus, a need exists for an improved method for transferring a layer from a source substrate to a support substrate when adhesive is used for joining the substrates.